The long term objective of the proposed work is to elucidate the mechanism of insulin action in human body. The specific aim of the current study is to explain the mechanism by which insulin stimulates sugar transport across the plasma membranes of muscle and fat cells. We plan to approach the problem in three steps. In the first step, we will study the general characteristics of the transport system in fat cells. In the second step, we will study the transport in vesicles prepared from plasma membranes. In the third step, we will attempt to reconstitute the hormone action in a cell-free system.